prevent suicide when bc(1) is envoked with bad args

Reference: /n/sources/patch/applied/bc-suicide
Date: Sun Oct 16 13:05:46 CES 2005

--- /sys/src/cmd/bc.y	Sun Oct 16 13:05:18 2005
+++ /sys/src/cmd/bc.y	Sun Oct 16 13:05:15 2005
@@ -796,7 +796,8 @@
 		ln = 0;
 		goto loop;
 	}
-	Bterm(in);
+	if (in)
+		Bterm(in);
 	if((in = Bopen(sargv[ifile], OREAD)) != 0){
 		ln = 0;
 		ss = sargv[ifile];